Diversification of neuronal subtypes is a common theme of central nervous system (CNS) development and is essential for its normal function. Furthermore, the distinct molecular profiles of different neuronal subtypes likely confer differential disease susceptibility and response to therapeutics. A thorough characterization of the molecular differences between subtypes is therefore crucial for understanding normal and pathological neuronal function. In addition, the molecular mechanisms driving diversification remain an open question in the field. This proposal utilizes avian photoreceptors as a model for understanding molecular heterogeneity between closely related neuronal subtypes, and for defining the mechanisms by which subtype diversification is established and maintained. The avian photoreceptors are an ideal model in which to study CNS diversification because they are a diverse class of neurons comprised of well-defined subtypes that can be successfully isolated, and because the developing avian retina is easy to access and manipulate. Because very few genes specific to avian photoreceptor subtypes are known, Aim 1 of this proposal is to define subtype specific gene expression by conducting high-throughput sequencing of photoreceptor subtypes isolated using FACS. In addition to providing candidate genes for dissecting the molecular mechanisms underlying the diverse functions of these cells, this approach will identify transcription factors specific to photoreceptor subtypes, an invaluable tool for exploring the molecular mechanisms driving diversification. The transcription factor spalt-like 1 (SALL1) is enriched in green single cones relative to rods (Aim 1, pilot study). Because spalt family members drive photoreceptor diversification in the mouse and fly, SALL1 is an excellent candidate for driving diversification in avian photoreceptors. The hypothesis that SALL1 directs green cone specification and differentiation will be tested in Aim 2 by in vivo knockdown and mis-expression in the developing retina. If SALL1 does direct green cone diversification, it will be the first transcription factor shown to drive avian photoreceptor diversification, a critical stp in exploiting this excellent model system for CNS diversification. This approach may then be extended to define a network of transcription factors involved in diversification and will promote further inquiry into the intersection of transcriptional regulation with chromatin remodeling and extracellular signaling. Exploring these mechanisms in avian photoreceptors will provide critical insights into CNS diversification and advance the development of novel therapies for retinal disease.